1. Field of the Invention
The present invention relates to a fluid operated booster for a vehicle brake system in which a control spool is operated in a switching mode in response to the operation of a brake pedal rod, hydraulic pressure provided by an accumulator is applied through the control spool thus operating a power piston, and power provided by the power piston is applied to a master cylinder piston, to activate the brake system.
2. Related Art
A fluid operated booster for assisting the operating power of a brake system has well been known in the art. One example of the conventional fluid operated booster as disclosed in U.S. Pat. No. 4,435,960 will be described with reference to FIG. 2.
In FIG. 2, reference numeral 51 designates a housing of the conventional fluid operated booster which consists of a first parts 52 and a second 53. In the booster, a boost piston 55 is slidably supported by an annular plug 56, which is disposed between the first and second parts 52 and 53 of the housing 51. The boost piston 55 is provided with an output part 62 and a control part 63 smaller in diameter than the output part 62. The output part 62 and the control part 63 are coupled together through a snap ring 68. A boost chamber 54 is formed in the control part 63, and a control spool 57 is arranged in the chamber 54. The boost chamber 54 is communicated through a through hole 64 with a hydraulic pressure chamber which applies hydraulic pressure to the output part 62. The control spool 57 performs to communicate the boost chamber 54 with a high pressure chamber 58 and a low pressure chamber 59. The high pressure chamber 58 is connected to an accumulator passageway 69, and the low pressure chamber 59 is connected to a passage 70 extending to a reservoir tank. The control spool 57 abuts against an input piston 60 by the elastic force of a spring 61 disposed in the boost chamber 54.
Upon application of an operating force to the input piston 60, the control spool 57 is moved against the elastic force of the spring 16 to the left in FIG. 2, so that the communication of the low pressure chamber 59 with the boost chamber 54 is interrupted, and then the high pressure chamber 58 is communicated with the boost chamber 54 through a hole formed in the control spool 57. As a result, the hydraulic pressure is applied through the boost chamber 54, the hole 64 and the hydraulic pressure chamber to the output part 62 of the boost piston 55, thus operating the master cylinder. This hydraulic pressure assists the operating power of the input piston 60.
When the hydraulic pressure in the boost chamber 54 increases to the extent that the reaction force on the control spool 57 is greater than a predetermined value, the control spool 57 is moved, together with the input piston 60, to the right in FIG. 2 by the hydraulic pressure of the boost chamber, thus interrupting the communication of the boost chamber 54 with the high pressure chamber and the low pressure chamber. When the operating force applied to the input piston 60 decreases, the input piston 60 and the control spool 57 are moved to the right in FIG. 2, to communicate the boost chamber 54 with the low pressure chamber 59, as a result of which the brake is released.
In the above-described fluid operated booster, the high hydraulic pressure is applied from the accumulator to seal members 67 and 66 provided at the output part 62 and the control part 63 of the boost piston. Hence, relatively high frictional resistance is provided between the housing 51 and the seal members 67 and 66, thus resisting the movement of the boost piston. That is, the resistance makes it difficult to smoothly move the boost piston 55. As for the seal member 66 of the control part 63, the high hydraulic pressure from the accumulator is applied to it at all times. Therefore, the seal member 66 is strongly pushed against the boost piston, so that the frictional resistance between the seal member and the boost piston is high, thus impeding the initial smooth movement of the boost piston. Furthermore, in the fluid operated booster, the control spool 57 is urged towards the input piston 60 by the spring 61, and therefore the booster is large in axial length, and rather intricate in structure, and it is difficult to miniaturize.
In order to eliminate the direct application of the hydraulic pressure from the accumulator to the seal members thereby allowing the smooth movement of the boost piston, Unexamined Japanese Utility Model Application (OPI) No. Hei. 1-147773 (the term "OPI" as used herein means an "unexamined published application") has proposed a fluid operated booster. However, the booster thus proposed is still disadvantageous in that the housing is bulky because the movement of the spool valve is transmitted through a link member to the power piston; that is, it is difficult to miniaturize the booster.